A conventional ultrasound imaging system comprises an array of ultrasonic transducers for transmitting an ultrasound beam and receiving a reflected beam from the object being studied. For ultrasound imaging, a one-dimensional array typically has a multiplicity of transducers arranged in a line and driven with separate voltages. By selecting the time delay (or phase) and amplitude of the applied voltages, the individual transducers can be controlled to produce ultrasonic waves which combine to form a net ultrasonic wave that travels along a preferred vector direction and is focused at a selected point along the beam. Multiple firings may be used to acquire data representing the same anatomical information. The beamforming parameters of each of the firings may be varied to provide a change in maximum focus or otherwise change the content of the received data for each firing, e.g., by transmitting successive beams along the same scan line with the focal point of each beam being shifted relative to the focal point of the previous beam. By changing the time delay and amplitude of the applied voltages, the beam with its focal point can be moved in a plane to scan the object.
The same principles apply when the transducer array is employed to receive the reflected sound energy (receiver mode). The voltages produced at the receiving transducers are summed so that the net signal is indicative of the ultrasound reflected from a single focal point in the object. As with the transmission mode, this focused reception of the ultrasonic energy is achieved by imparting a separate time delay (and/or phase shift) and gain to the signal from each receiving transducer.
A phased-array ultrasound transducer is made up of an array of small piezoelectric transducer elements, with an independent electrical connection to each element. In most conventional transducers the elements are arranged in a single row, spaced at a fine pitch (one-half to one acoustic wavelength on center). As used herein, the term "1D" transducer array refers to a single-row transducer array having an elevation aperture which is fixed and an elevation focus which is at a fixed range. Electronic circuitry coupled to the elements uses time delays, and sometimes phase rotations, to control the transmitted and received signals and form ultrasound beams which are steered and focused throughout the imaging plane. For some ultrasound systems and probes, the number of transducer elements in the probe exceeds the number of channels of beamformer electronics in the system. In these cases an electronic multiplexer is used to dynamically couple the available channels to different (typically contiguous) subsets of the transducer elements during different portions of the image formation process.
A typical 1D linear or convex transducer array and multiplexer may operate with 128 beamformer channels, but the transducer array itself may have significantly more elements. A multiplexer can allow a set of contiguous transducer elements to be simultaneously coupled to the beamformer channels via coaxial cables. By closing switches coupled to respective transducer elements in a set, the beamformer may be coupled to one end of the transducer array and focused beams of ultrasound may be transmitted and received to acquire data for the corresponding edge of the image. As the point of origin of successive ultrasound beams steps along the transducer array toward the opposite end, it becomes advantageous to shift the active aperture so that the origin of the ultrasound beam is centered within it. To shift the aperture from the extreme left end of the array, for example, by one element toward the opposite end, the multiplexer switch coupled to the first element is opened and the switch coupled to the last element in a subsequent set is closed. This shifts the first beamformer channel from the one end to the opposite end of the active aperture, while leaving all other channels and elements connected as before. The time delays and other beamforming parameters are changed by the software to correspond to the new multiplexer state and one or more additional image vectors are acquired. Then the aperture is stepped further to the right, by opening the switch coupled to the second element and closing the switch coupled to the next element in the subsequent set. In this manner the active aperture can be stepped sequentially from one end of the transducer array to the other.
Various types of multi-row transducer arrays, including so-called "1.25D", "1.5D", "1.75D" and "2D" arrays, have been developed to improve upon the limited elevation performance of 1D arrays. As used herein, these terms have the following meanings: 1.25D) elevation aperture is variable, but focusing remains static; 1.5D) elevation aperture, shading, and focusing are dynamically variable, but symmetric about the centerline of the array; 1.75D) elevation geometry and control are similar to 1.5D, but without the symmetry constraint; and 2D) elevation geometry and performance are comparable to azimuth, with full electronic apodization, focusing and steering. The elevation aperture of a 1.25D probe increases with range, but the elevation focusing of that aperture is static and determined principally by a mechanical lens, with a fixed focus (or fixed foci). 1.25D probes can provide substantially better near-field and far-field slice thickness performance than 1D probes, and require no additional system beamformer channels. 1.5D probes use additional beamformer channels to provide dynamic focusing and apodization in elevation. 1.5D probes can provide detail resolution comparable to, and contrast resolution substantially better than, 1.25D probes, particularly in the mid-field and far-field. 1.75D probes, with independent control of the beamforming time delays for all elements in the aperture, allow the beamformer to adaptively compensate for inhomogeneous propagation velocities in the body (or nonuniformities in the imaging system or transducer). In addition to such adaptive beamforming or phase aberration control, 1.75D probes may also support limited beam steering in the elevation direction.
By providing at least apodization (1.25D) and perhaps dynamic beamforming (1.5D), phase aberration control (1.75D), or full 2D beam steering, multi-row transducer arrays significantly improve upon the limited elevation performance of 1D probes. However, as the number of elements in the transducer increases, the number of channels in the beamformer is not keeping pace, and the function of the multiplexer is increasingly important.
U.S. Pat. No. 5,520,187 to Snyder, the disclosure of which is incorporated by reference herein, while not discussing multi-row transducer arrays, describes a flexible multiplexer which supports different multiplexer states for systems with different numbers of beamformer channels. The multiplexer states can be reprogrammed by the ultrasound imaging system, e.g., via a serial interface. These features are advantageously used in multi-row array multiplexers such as those disclosed hereinbelow.
U.S. Pat. No. 5,329,930 to Thomas and Harsh, the disclosure of which is incorporated by reference herein, discloses a method of synthetic aperture imaging, whereby a finite number of system beamformer channels are coupled via a multiplexer to successive subsets of a large number of transducer elements. For each desired image vector and for each subset of the transducer elements, an acoustic beam is transmitted, received and summed coherently with the acoustic data from the other subsets of the transducer elements. In this way an N-channel beamformer can achieve most of the resolution and signal-to-noise performance available from an (M.times.N)-element transducer, albeit at the cost of M transmit-receive cycles per resulting image vector (hence the name 1-for-M or 1:M imaging). Thomas and Harsh discuss criteria for the multiplexer but do not disclose any specific design for it. The multiplexer disclosed hereinbelow satisfies the criteria of U.S. Pat. No. 5,329,930 and is designed to support 1:M beamforming of 1.5D and 1.75D transducer arrays.